Production of ethanol by microorganisms provides an alternative energy source to fossil fuels and is therefore an important area of current research. It is desirable that microorganisms producing ethanol, as well as other useful products, be capable of growing and producing ethanol in a medium that does not impact the human food supply, such as avoiding use of sugars produced from corn grain. As a result of developments in cellulosic biomass processing, glucose, xylose, and other sugars may be released in high concentrations in a biomass hydrolysate for use in fermentation. As such, conversion of biomass to ethanol poses great possibility for improving environmental impacts by using renewable non-food resources to provide an alternative to fossil fuels.
Zymomonas mobilis and other bacterial ethanologens which do not naturally utilize xylose have been genetically engineered for xylose utilization to improve growth and ethanol production by using more of the sugars in biomass hydrolysate. However, growth and ethanol production in biomass-hydrolysate containing medium is typically not optimal due to the presence of acetate and other compounds that are inhibitory to microorganisms. Disclosed in commonly owned and co-pending United States Patent Publication US20110014670A1 is a method for producing an improved xylose-utilizing Zymomonas strain that is more tolerant to acetate and ethanol in the medium, as well as strains isolated by the method.
The toxic effect of single compounds likely to be found in the hydrolysates of pretreated biomass is described in Delegenes et al. ((1996) Enzymes and Microbial Technology 19:220-224). Adaptation of xylose-fermenting Zymomonas mobilis to conditioned dilute acid yellow poplar hemicellulose hydrolysate is described in Lawford et al. ((1999), Applied Biochemistry and Biotechnology 77:191-204).
There remains a need for isolated xylose-utilizing Zymomonas ethanologen strains with improved ethanol production during fermentation in biomass hydrolysate medium, and methods for genetic engineering to produce improved strains.